I just got the yen to experiment with a subwoofer out
on my Yamaha receiver.

There's a couple 8 inchers I haven't built Voigt pipes for yet.
I'm wondering if I can do a quick and dirty build with one
or both.

I have the RP U100 book out in front of me, but sure can't see
anything about a cutoff frequency or what goes on inside the Yam.

Oh yeah, no spec sheet on these 2 Roland 8 inchers either.
They were sold as closeouts from Roland pianos on Ebay.

Would it be a waste of time to mount these? Play with
isobarik maybe?

I'm on my way to do one of Fitz's Tuba 18's eventually.